1. Field of the Invention
The technology disclosed relates to a controller and a control method for a motorised vehicle, and in particular to a controller for a motorised vehicle having at least a left wheel and a right wheel, the left wheel and the right wheel being driven independently by a motor arrangement. In accordance with such motorised vehicles, turning of the vehicle is typically effected by driving the left wheel and the right wheel at different speeds. A typical example of such a motorised vehicle is an electric wheelchair.
2. Description of the Prior Art
Conventional electric wheelchairs have at least a left wheel and a right wheel which are driven independently by a motor arrangement. Whilst the motor arrangement used to drive the wheels may comprise a single motor with independent couplings between the motor and the wheels to effect independent drive of those wheels, it is more common that each driven wheel is driven by an independent motor. Often the electric wheelchair will include two or more castors which rotate to follow the direction of travel, and optionally a number of further fixed wheels may be provided. The speed and direction of each driven wheel defines a linear forward/reverse speed and turning (spin) rate of the wheelchair.
A typical input control interface for an electric wheelchair provides a joystick having two control axes, one to control the forward/reverse speed and one to control the turn rate (or spin speed). However, it will be appreciated that a joystick is not a requirement, and many other types of input control interface have been developed to allow control by users with a variety of different disabilities, for example, head control based mechanisms, sip puff based mechanisms and so on.
In providing the user of an electric wheelchair with control over its speed and spin, it is nonetheless common to impose certain limits on combinations of speed and spin which the user can request. This is done essentially for safety reasons, because for example a maximum spin speed (with the left and right wheel being driven in opposite directions) may be simply requested when the wheelchair is stationary (to thus pivot on the spot), but such a sharp turn would be dangerous if requested when the wheelchair is already travelling with substantial linear speed, in the worst case scenario causing the wheelchair to overturn. It is therefore known to impose “gate shaping” on the control interface, which typically allows a greater degree of spin to be requested in combination with low linear speeds, whilst limiting more restrictively the spin that may be requested in combination with high linear speeds. Such gate shaping is typically implemented by means of an algorithm which transforms signals from the user's control interface into signals which control the motors of the wheelchair.
Even without entering a regime where the wheelchair is liable to tip over, management of the speed and spin of the wheelchair is still necessary because of the effect that certain combinations of speed and spin can have on the traction of each wheel during a turn. A loss of fraction of one wheel can have undesirable consequences for the handling of the wheelchair when making a turn. One reason why traction may be lost is the centripetal force acting on the wheelchair during a turn, which will tend to lift the inside wheel thus reducing its traction. This problem is worse with front wheel drive wheelchairs since the inside wheel is the high torque wheel and is thus more likely to lose traction. If a significant degree of transaction is lost on the inside wheel, then the wheelchair may be liable to spin out of control into the turn. Even without such drastic lost of control, even a smaller loss of traction may cause handling difficulties for the user of the wheelchair, since the turn radius which the wheelchair follows will not correspond to that requested by the operation of the user controls.
Conventional gate shaping approaches to this problem can certainly prevent the wheelchair from overturning, but nevertheless suffer from problems in the handling characteristics of the wheelchair due to the imposed limits on the relative speed and spin affecting the smooth steering feel of the wheelchair, and have led to the overly severe inhibiting of the spin speed in certain joystick demand regions.
Some background technological information to the technology disclosed can be found in the “enAble40 Powerchair Control System” manual produced by Curtis Instruments, Inc. of New York, USA; in U.S. Pat. Nos. 5,033,000 and 5,307,888; and in US Patent Application Publication 2010/0007299 A1.
It would be desirable to provide an improved technique for controlling motorised vehicles which would alleviate the above-discussed problems.